(1) Field of the Invention
This invention relates to a nonvolatile semiconductor memory. More particularly, the invention relates to a memory comprising a plurality of semiconductor devices having a memory function, the memory being capable of writing and erasing information electrically.
(2) Description of the Prior Art
As a nonvolatile semiconductor memory capable of writing and erasing information electrically (hereinbelow, termed "EAROM" which is short for "electrically alterable read only memory"), there has been known a device wherein metal-nitride-oxide-semiconductor field-effect transistors (hereinafter, abbreviated to "MNOS Tr's") are arrayed in the form of a matrix (J. T. Wallmark et al: International Electron Devices Meeting, Abstract page 140 (1968, Oct.)). In the MNOS Tr, electrons and holes are injected into traps in the interface between two kinds of insulating films (Si.sub.3 N.sub.4 film and SiO.sub.2 film) through the thin SiO.sub.2 film from an Si side by the tunnel effect. FIG. 1 shows a sectional view of a prior-art MNOS Tr. As illustrated in the figure, the MNOS Tr having hitherto been known is of the P-channel type and employs a gate electrode of aluminum (Al). Referring to the figure, numeral 11 designates a silicon (Si) body of N-type conductivity, numerals 12 and 13 diffusion layers of P.sup.+ -type conductivity to serve as source and drain regions respectively, numeral 14 an SiO.sub.2 film, numeral 15 an Si.sub.3 N.sub.4 film, and numeral 16 an Al electrode to serve as a gate electrode. The EAROM which uses the prior-art P-channel MNOS Tr's employing the Al gate as shown in FIG. 1 has many disadvantages as follows:
(1) The operating speed is very low. PA1 (2) The bit capacity is small. PA1 (3) The number of read accesses after writing data is subject to limitation, and when the data are read out approximately 10.sup.7 -10.sup.11 times, the disappearance of the data takes place. PA1 (4) The unpowered nonvolatile data storage time of the memory is short. PA1 (1) the integration density is high, PA1 (2) the operating speed is high, PA1 (3) the number of read accesses is not limited, and PA1 (4) the nonvolatile data storage time is long, as well as a nonvolatile semiconductor memory device which can be constructed with such an EAROM.
(5) The number of write-erase cycles is subject to limitation, and is approximately 10.sup.4 -10.sup.6.
The disadvantage (5) results from the employment of the MNOS Tr itself, and drastic improvements are difficult. As regards the items (1) to (4), the enhancement of performance is desired, but any especially effective betterment has heretofore been made.